"P" compasses were the usual types throughout the war, and for some time after. If there are no other markings (usuall handwritten in yellowish paint) anywhere, it's entirely possible that it still contains the original radium paint and powder, which made it visible in the dark. If the cardinal points and/or the wires under the glass of the rotating top appear to be a mucky brown, possibly with black specks, don't remove the top, but get it checked out with a geiger counter ASAP. Before anyone starts on about how the amount of radiation is minimal, and therefore harmless, old radium paint turns to dust, so, breathe it in, and.............. The interior consists of an eight-point "spider" framework, with a central pivot, which sits on a post, in which there's a sapphire, which has a slight depression in it, which keeps the pivot (and hence the spider) central. The north, east & west arms of the spider should have sealed glass tubes, containing radium powder, attached to them, with an extra short cross-tube on the north wire. There should be another, very short, tube attached near the edge of the bowl, under the glass; this was aligned with the a/c centre line/nose. The interior should be completely black, and filled with 100% industrial alcohol (poisonous & foul-tasting) or a 90%/10% mix of alcohol/water. I can't remember the P8, specifically, but some were meant to operate upside-down, and be viewed through a mirror (on the C-47, for example,) which means that the lettering is back-to-front, and east and west appear to be on the wrong sides. The compass had a large bowl, since it had a permitted "dip," of the spider, of around +/- 7 degrees, which meant that it could be used anywhere in the world (nowadays, if a compass is balanced for use in the northern hemisphere, it will need rebalancing for the southern.)Edgar

Normally, M stands for modified, and the only major mod, that I can think of, concerns the luminosity. At some stage, the radium was replaced by fluorescent powder and paint, and the only way to check is to take the compass into a darkened room. If there's no sign of any glow, it could be the age of the paint, but try illuminating it with ultra-violet lighting, and, if the spider tubes, crosswires, and cardinal points show up, clearly, your compass has been altered, and shouldn't be any problem.The 143617 B is probably the serial no., with the B indicating the manufacturing company. 6A/726 is the stores reference no; later (post-war) compasses were 6B/34 and 6C/2027, for example. AM is Air Ministry, and there was normally an arrow-head, somewhere, as well. Sorry, should have told you this, before, but forgot.Edgar